The present invention relates to the production of a UV stable lubricating base oil from a waxy hydrocarbon feedstock involving the steps of hydrocracking a heavy hydrocarbon feedstock to prepare a waxy hydrocarbon feedstock, dewaxing the waxy hydrocarbon feedstock to produce a lubricating base oil, and hydrofinishing the lubricating base oil to produce a UV stable lubricating base oil.
The preparation of lubricating base oils from heavy hydrocarbon feedstocks using the sequential steps of hydrocracking the heavy hydrocarbon feedstock to prepare a waxy intermediate feedstock, catalytically dewaxing the intermediate feedstock to produce a lubricating base oil, and hydrofinishing the lubricating base oil to obtain a UV stabilized lubricating base oil product is well known and is generally described in U.S. Pat. No. 4,283,272. In upgrading hydrocarbon feedstocks to produce a suitable lubricating base oil the lubricating properties of the material must be improved, i.e., the base oil should display a high viscosity index, high thermal stability, oxidation resistance, and a high boiling range. As used in this disclosure the term heavy hydrocarbon feedstock refers to a hydrocarbon boiling above about 650 degrees F. (340 degrees C.) which is suitable for upgrading to a lubricating base oil. As used herein, heavy hydrocarbon feedstock, includes, but is not limited to, petroleum derived feedstocks, such as for example heavy straight run gas oil, deasphalted oil, vacuum gas oil, topped crude oils, atmospheric residuum, or the like. Also useful as possible feedstocks are synthetic hydrocarbons prepared from shale oils, coal, or by Fischer-Tropsch processes. However, feedstocks which are high in asphaltenes, metals, sulfur, and nitrogen will usually require some kind of prior treatment, such as in a hydrotreating operation, before they are suitable for use as a feedstock for the hydrocracking process step. Hydrocracking is a well known process for upgrading hydrocarbon feedstocks for use in the manufacture of lubricating base oils. Hydrocracking operations take place under severe hydrogenation conditions in the presence of excess free hydrogen and a catalyst having good hydrogenation activity. Typically hydrocracking is carried out at temperatures of from about 500 degrees F. (260 degrees C.) to about 900 degrees F. (480 degrees C.), preferably within the range of from about 650 degrees F. (340 degrees C.) to about 800 degrees F. (425 degrees C.). The literature teaches that the pressures in the hydrocracking zone are within the range from about 500 psig to 10,000 psig with the range from about 500 psig to about 3000 psig being preferred. In commercial operations the pressure is almost always in excess of 1500 psig. The hydrogen supply rate (both makeup and recycle) falls within the range of from about 500 to 20,000 standard cubic feet (SCF) per barrel of hydrocarbon feed, preferably in the range from about 2000 to 10,000 SCF. per barrel of hydrocarbon feed. See U.S. Pat. No. 3,852,207. Most hydrocracking operations produce a number of useful products which include transportation fuels, such as jet, kerosene, and naphtha, as well as the feedstocks suitable for upgrading to lubricating base oils. The feedstocks which are suitable for further processing into lubricating base oils are often high in paraffins. Such feedstocks referred to in this disclosure as waxy intermediate feedstocks contain at least 5 percent by weight total wax and, in some cases, may contain greater than 80 percent total wax as in the case with slack wax, deoiled wax, or synthetic liquid polymers. However, usually the feedstock will contain at least 10 percent by weight of wax.
Waxy intermediate feedstocks produced by the hydrocracking operation are characterized by high pour points and high cloud points. In order to prepare commercially useful lubricating base oils from waxy feedstocks the pour point and cloud point must be lowered without compromising the desired viscosity characteristics. Dewaxing operations may employ either solvent dewaxing or catalytic dewaxing processes to improve the lubricating characteristics of the feedstock. The present invention is concerned only with catalytic dewaxing processes, and, more particularly, the present invention is directed only to those catalytic dewaxing processes which employ an isomerization-type of catalyst such as is described in U.S. Pat. Nos. 5,135,638; 5,282,958; and 5,376,260. In isomerization-type dewaxing operations the cracking of the paraffinic components of the hydrocarbon is minimized. The straight chain and slightly branched paraffins are isomerized to more highly branched materials which have more desirable pour point characteristics. By contrast, those catalytic dewaxing operations using a cracking type of catalyst (such as described in U.S. Pat. Nos. 3,894,938; 4,176,050; 4,181,598; 4,222,855; 4,229,282; and 2,247,388) crack the straight chain and slightly branched paraffins into smaller molecules which are subsequently removed. In commercial operations, the dewaxing operation is carried out at a relatively high pressure. Typically in commercial lube production plants, the hydrocracking operation, the catalytic dewaxing operation and the hydrofinishing operation are carried out at substantially the same total pressure, usually in the range of from about 1500 psig to about 2500 psig. However, it has been reported that in isomerization-type catalytic dewaxing operations where a silicoaluminophosphate molecular sieve is used there is improved selectivity at lower total pressures than at those pressures normally used in present commercial operations. See U.S. Pat. No. 5,082,986.
Hydrofinishing operations, such as described in U.S. Pat. Nos. 3,852,207 and 4,673,487, are intended to stabilize the lubricating base oil recovered from the dewaxer. The optimal pressure for carrying out hydrofinishing operations is relatively high, usually above about 1500 psig and more preferably above 2000 psig.
As used in this disclosure the term UV stability refers to the stability of the lubricating base oil when exposed to ultraviolet light and oxygen. Instability is indicated when the lubricating base oil forms a visible precipitate or darker color upon exposure to ultraviolet light and air which results in a cloudiness or floc in the product. Usually lubricating base oils prepared by hydrocracking followed by catalytic dewaxing require UV stabilization before they are suitable for use in the manufacture of commercial lubricating oils.
The present invention is directed to a processing scheme which makes use of the optimal conditions of each of the operations involved in the production of lubricating base oils.
In its broadest aspect, the present invention is directed to a process for producing a lubricating base oil having good UV stability from a waxy hydrocarbon feedstock which comprises dewaxing the waxy hydrocarbon feedstock in an isomerization zone in the presence of an isomerization catalyst under isomerization conditions at a total pressure of less than 1500 psig to produce a lubricating base oil product having improved lubricating base oil properties as compared to the waxy hydrocarbon feed; recovering from the isomerization zone a lubricating base oil product and light materials; increasing the pressure of the lubricating base oil product mixture to a total pressure of greater than 1500 psig; stabilizing the lubricating base oil product in a hydrofinishing zone in the presence of a hydrofinishing catalyst and hydrogen under hydrofinishing conditions at a total pressure in excess of 1500 psig to produce a UV stabilized lubricating base oil mixed with hydrogen rich off-gas; separating the UV stabilized lubricating base oil from the hydrogen rich offgas; and recovering the UV stabilized lubricating base oil. The light materials referred to as part of the mixture recovered from the isomerization zone include excess hydrogen and lighter hydrocarbons such as naphtha and the like, usually referred to as hydrogen rich off-gas, having a boiling range below that of the lubricating oil product.
The present invention is particularly useful when employed in association with a hydrocracking operation. In a preferred embodiment, the invention is directed to an integrated process for producing a lubricating base oil having good UV stability from a heavy hydrocarbon feedstock which comprises hydrocracking the heavy hydrocarbon feedstock to hydrocrackate products in a hydrocracking zone in the presence of a hydrocracking catalyst and hydrogen under hydrocracking conditions wherein said hydrocracking conditions and hydrocracking catalyst are preselected so that at least a fraction of the hydrocrackate products comprise a waxy intermediate feedstock having an initial boiling point above 650 degrees F. (340 degrees C.) and an end boiling point below the end boiling point of the heavy hydrocarbon feedstock; separating the waxy intermediate feedstock from the other hydrocrackate products produced in the hydrocracking zone and passing the waxy intermediate feedstock to a dewaxing zone; dewaxing the intermediate feedstock in a isomerization zone in the presence of a isomerizing catalyst and hydrogen under isomerization conditions at a pressure that is substantially less than the pressure present in the hydrocracking zone to produce a lubricating base oil product having improved lubricating base oil properties as compared to the waxy hydrocarbon feedstock; stabilizing the lubricating base oil product in a hydrofinishing zone in the presence of a hydrofinishing catalyst and hydrogen under hydrofinishing conditions at a pressure substantially greater than the pressure in the dewaxing zone to produce a UV stabilized lubricating base oil mixed with hydrogen rich off-gas; separating the UV stabilized lubricating base oil from the hydrogen rich off-gas in a high pressure separator; routing the hydrogen rich off-gas from the high pressure separator into the hydrocracking zone; and recovering the UV stabilized lubricating base oil. The hydrocracking zone operates at similar pressure to the high pressure hydrofinishing zone.
In carrying out the present invention, the hydrocracking and the hydrofinishing operations will usually be carried out at a total pressure in excess of 1500 psig, and preferably will be carried out at a pressure of at least 2000 psig. In carrying out the present invention the hydrocracking and hydrofinishing operations will usually be conducted at similar total pressure. However, the dewaxing operation will usually be carried out at a total pressure below 1500 psig and preferably at a pressure below about 1000 psig.